Mayor Kurt L. Schmoke threatened yesterday to veto a bill being promoted by two firefighters unions that would require four-person crews on fire engines, saying that passage of the bill would force him to close firehouses as a cost-cutting measure.

The bill was drafted in June after firefighters persuaded council members that staffing fire engines with fewer than four people presented a safety risk to firefighters and city residents alike. It was given preliminary approval by the Baltimore City Council Monday and final passage was expected at the council's next meeting this coming Monday.

The mayor said that four-person engine crews are indeed preferable, and that he will require fire officials to maintain them whenever possible after Dec. 8, when another 59 firefighters are to join the force.

But he also said he would allow fire officials to field three-member crews when the department faced temporary staff shortages, such as when firefighters unexpectedly call in sick.

"It's imperative for us to make clear we are moving to the four-person staff rate," the mayor said. "But we also want to make sure the chief has the flexibility to deal with emergency situations."

Several council members expressed annoyance that the mayor would threaten the council with a veto before exhausting efforts to reach a compromise.

"That's the art of politics, but I guess he wants to go to the mat on this one," said Councilman Joseph J. DiBlasi, D-6th, the bill's sponsor.

Mr. DiBlasi said a law requiring the minimum staffing levels would be appropriate because the mayor has already failed to deliver on promises regarding staffing levels.

"They said we'd have the staffing in September, then November, then December. How long can we wait?" Mr. DiBlasi said. "It's a safety issue not only for the firefighter but the citizens."

Mr. Schmoke's press secretary, Clint Coleman, said the mayor has used his veto power only once before, in 1988, to strike down a bill that would have forced him to reopen firehouses that had been shut as a cost-cutting measure.

The city, which traditionally has operated with four-person crews, began dispatching the smaller crews June 3 because of a reduction in the firefighter workweek as mandated by a labor agreement, according to fire spokesman Capt. Patrick P. Flynn.

The workweek cut, coupled with rule changes that allowed firefighters to use vacation time to extend their weekends, forced fire administrators to chose between sending out smaller crews or authorizing $750,000 per month in extra overtime, Captain Flynn said.

He said the city has been able to return to four-member crews on weekdays since 62 fire academy graduates joined the force in September. He said that with the additional 59 graduates expected Dec. 8, the department should be able to field full crews.

"But there are a lot of 'ifs,' " Captain Flynn said. "If we are hit with a lot of sickness or if the men utilize their vacation on the weekends, we could be short."

The mayor also said that the Police Department will begin its annual deployment of extra officers for the holiday season two weeks earlier than normal this year because of indications that the slowing economy is resulting in an increase in burglaries, purse snatchings and other crimes that generally rise during the holiday season.

The extra officers will target 14 neighborhoods around the city until Jan. 2, according to the mayor, who said he would not identify which neighborhoods to avoid tipping off the criminals.